The present disclosure is directed generally to methods, systems, and apparatus for changing characteristics of noise and, more particularly, to methods, systems, and apparatus for changing characteristics of noise by attenuating combustion noise arising from premixed flames.
Combustion driven oscillations are a form of combustion instability that typically occurs in premixed flames. Such instability is normally manifested in the form of noise, such as howling. Howling is generally considered to be unacceptable noise that is usually manifested in a range of 100-1000 Hz and at intensities up to about 120 dB.
Howling may be transient or continuous. Typically, transient howling occurs upon ignition of the flame and disappears after a relatively brief time (e.g., several seconds or minutes). Transient howling may be considered unacceptable. In contrast, continuous howling may continue as long as the burner is operational. Continuous howling is considered far more unacceptable. There is little known theory explaining the types of combustion oscillation that cause howling, particularly in industrial-type ribbon burners.
It is known that a variety of operating conditions may contribute to the formation of howling. For example, any specific burner construction may have a range of operating conditions in which combustion oscillations, which produce howling, may occur. In this regard, some important variables that can have an impact on this range of operating conditions are the flame power (i.e., fuel type and flow rate thereof) and burner-to-backing roll separation (i.e., “burner gap”). Unfortunately, the operating conditions at which combustion oscillations can occur for some burners are those which provide optimal burner performance.
As noted, flame stability controls formation of the combustion noise. More stable flames will have fewer tendencies to generate audible oscillations. Preheating the combustible mixture entering the burner ports tends to stabilize the flame at the attachment point to the ribbons. Fluctuations in the attachment of the flame to the ribbon initiate a pressure wave that enables the formation of combustion noise. On the other hand, impinging a flame on the chill roll surface serves to intensify a pressure wave, thus increasing the propensity to form and/or intensity of the combustion noise. Moreover, in the context of oxygen-enriched flames, the laminar flame speed and flame temperatures are greatly increased, thereby increasing the likelihood of howling. As a result, oxygen-enriched flames represent the most challenging condition for the elimination of combustion instability encountered with ribbon burners.
Without the ability to control unacceptable noises associated with operation of burners, such as ribbon burners, a relatively unhealthy and unpleasant working environment may exist. While known approaches have been undertaken to attenuate combustion noises by improving upon flame stability of burners, continuing efforts are, nevertheless, being undertaken to improve over known techniques.